Electrochromic devices

ABSTRACT

An electrochromic device having successive layers of electrochromic electrolyte and counter-electrode materials. The counter-electrode material comprises an oxide of a mixture including at least two of vanadium, titanium and zirconium.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to electrochromic devices as used, for example, inso-called variable transmission windows or variable reflection mirrors,and in particular to counter-electrode materials for such devices.

2. Background Art

Electrochromic devices are known to have successive layers ofelectrochromic, electrolyte and counter-electrode materials. The devicemay have first and second laminar substrates each covered on one sidewith an electrically conducting film, the layers interposed between thetwo substrates with the film covered sides innermost. Alternatively, thedevice may have one laminar substrate covered on one side with anelectrically conducting film, the layers being carried on this filmcovered side with a further electrically conducting film applied overthe exposed layer. The most common substrate material is glass, butplastics materials, like acrylic, may also be used.

By way of example, the electrically conductive films may be indium dopedtin oxide, the electrochromic material may be tungsten trioxide, thecounter-electrode material may be cerium titanium oxide and theelectrolyte material may be a suitable polymer to which lithiumperchlorate has been added.

A tungsten trioxide/cerium titanium oxide device can be changed betweenbleached and colored states by altering the applied electricalpotential, that is, the potential applied via the electricallyconductive films (acting as electrodes) across the electrochromic,electrolyte and counter-electrode layers. The polarity of the potentialdictates the direction of transfer of cations (provided by the lithiumperchlorate) through the electrolyte material, between theelectrochromic and the counter-electrode materials. The cation transferis reversible. When reduced, or in other words when cations areinserted, the electrochromic material is colored, whereas, when oxidized(when cations are de-inserted), it is virtually colorless. Conversely,the counter-electrode material is chosen because it is virtuallycolorless when either reduced or oxidized, or at least any coloring onreduction is indiscernible.

A tungsten trioxide/cerium titanium oxide device can be varied from ablue colored state to a pale yellow “colorless state”.

Other electrochromic/counter-electrode material combinations may work inreverse, with the electrochromic layer coloring on oxidation, anddifferent combinations can produce different colors and degrees of colorchange. There are also devices wherein a single layer acts as both thecounter-electrode and the electrically conducting film. Furthermore,there are devices, such as those available from the Gentex company,which have a single material which functions as the electrochromic,counter-electrode and electrolyte layers.

The changeability of an electrochromic device lends itself to use in,amongst other applications, a window where variable transmissioncharacteristics are required. These are seen as being of particular usein integrated energy management systems for buildings; one idea being tomodulate the solar gain of the building to maximize energy benefits. Forinstance, by coloring the window during the hottest part of a summer'sday, the amount of solar radiation entering a building can be minimized,and on dull winter days the window can be bleached so as to make bestuse of the available natural light.

The degree of coloration of an electrochromic device is related to thequantity of cations inserted into the electrochromic material and hence,in the case of an electrochromic material coloring on the insertion ofcations, the extent of reduction, which is dictated by the amount ofcharge passed; the greater the charge passed, the deeper the color. Oneof the limiting factors on the amount of charge passed is the chargestorage capacity of the counter-electrode material. For instance, in adevice which has a tungsten trioxide electrochromic layer and a ceriumtitanium oxide counter-electrode layer, the depth of the blue colorationattainable is restricted by the tendency of the cerium titanium oxide tosaturate at a charge density well below that which the tungsten trioxidecan tolerate. Thus, the tungsten trioxide effectively has unutilizedcharge storage capacity.

In addition, the dynamic range of the device, that is the ratio of thecolored to bleached state optical transmission, is preferably as wide aspossible. Most effective use of a management system controlledelectrochromic window is achieved if the device has as wide a dynamicrange as possible. Optimization of the dynamic range is assisted byhaving the counter-electrode layer as near as possible equally opticallytransmitting in both the colored and bleached states of the device.

A counter-electrode material also needs to have good long term cyclingstability and good electrochemical kinetics.

WO 89/12844 (EIC Laboratories Inc) discloses a counter-electrodematerial composed of a mixture of metal oxides in combination with anelectrochromic material coloring on reduction. Proposed in WO 89/12844are mixed oxides of materials such as vanadium or chromium eithertogether or with oxides of niobium, tantalum or titanium. However, WO89/12844 is directed to counter-electrode materials which complement theelectrochromic material, that is to say, counter-electrode materialsthat are colored when oxidized and colorless when reduced, and thespecific examples deal only with niobium/vanadium or chromium/vanadiumoxides and their characteristics.

SUMMARY OF THE INVENTION

The invention provides an electrochromic device having successive layersof electrochromic, electrolyte and counter-electrode materials,characterized in that the counter-electrode material comprises an oxideof a mixture including at least two of vanadium, titanium and zirconium.

The counter-electrode materials according to the invention have beenfound to have a significantly increased charge storage capacity, incomparison to, for example, cerium titanium oxide, which in turnfacilitates the utilization of the maximum charge storage capacity ofthe electrochromic material. Furthermore, the counter-electrodematerials according to the invention have been found to have goodoptical density characteristics. A significant factor in this is thatthe counter-electrodes used in devices according to the inventionexhibit minimal coloring in the “bleached state” of the device, thusmaximize the optical transmission difference between the bleached tocolored states. What is more, devices according to the invention arecapable of high electrochromic efficiency, which is a measure of thechange in optical density with charge. The potentially high overallelectrochromic efficiency of the device is a result of the relativelylow electrochromic efficiency of the counter-electrode material whichwill not therefore detract from the high electrochromic efficiency ofwhatever electrochromic material is used. The higher the electrochromicefficiency, the greater the optical density change for the quantity ofcharge passed. Hence, it is desirable to have as high an electrochromicefficiency as possible so as to bring about the maximum color change forthe minimum amount of charge. The less the charge required, the quickerand cheaper the device is to run. Dynamics and cost are both importantconsiderations for building energy management systems. However, thegreater the charge capacity of the counter-electrode, the greater theopportunity for taking advantage of any high electrochromic efficiencyof the device.

In addition, the counter-electrode materials according to the inventionhave been found to be electrochemically and mechanically stable and toenable faster preconditioning of the device (the process of initiatingcation transfer by cyclically driving the device between predeterminedpositive and negative voltages).

The counter-electrode material according to the invention is coloredwhen reduced and bleached when oxidized.

The mixture may include two of vanadium, titanium and zirconium in apercentage molar ratio of between 10:90 and 90:10. Preferably, themixture includes two of vanadium, titanium and zirconium in a percentagemolar ratio of 80:20, 60:40, 50:50, 40:60 or 20:80.

The counter-electrode layer may be between 100 and 10×10^(3 Å) thick.Preferably, the counter-electrode layer is between 1×10³ and 3×10³ Åthick.

The counter-electrode layer may be deposited by sputtering, preferablyreactive dc magnetron sputtering, or any other suitable method forapplying thin films to a substrate such as glass. The phrase “dcsputtering” as used herein means sputtering of single, dual or metaltargets using an applied dc voltage. Preferably, the sputtering iscarried out in an argon atmosphere containing between 0 and 100 volume %oxygen, at a total pressure of between 5×10⁻⁴ and 0.1 mbar. Furtherpreferably, the atmosphere contains between 10 and 50 volume % oxygen,at a total pressure of between 5×10⁻³ and 5×10⁻² mbar. However, as willbe appreciated, the most suitable sputtering conditions will be largelydependent upon the particular sputtering apparatus used. In the case ofa single target sputtering, the target may be an intermetalliccomprising metals melted together, a sintered or mixed metal oxide orany other suitable composition.

It has been found that higher charge storage capacities are obtained forhigher deposition pressures and lower oxygen concentrations.

Most preferably the mixture includes vanadium and titanium. The mostpreferred mixture may be deposited by sputtering, in which case thepercentage of oxygen admitted to and the pressure of the sputteringatmosphere and the thickness to which the counter-electrode layer isdeposited may be selected to provide the required charge capacity. Thevanadium content of the most preferred mixture may be selected toprovide the required electrochromic efficiency for the device. Mixtureswith a higher vanadium content show a reduction in their electrochromicefficiency as the amount of charge entered into the counter-electrodelayer (charge density) is increased. On the other hand, the mixtureswith a lower vanadium content show an increase in electrochromicefficiency as the charge density is increased. Mixtures between theseextremes show an virtually unvarying electrochromic efficiency. Titaniumin the counter-electrode layer appears to provide chemical stability;vanadium oxide is water sensitive and on its own is difficult to handle,but the addition of titanium adds stability to the mixture andsignificantly diminishes water sensitivity. The choice of the relativeproportions of vanadium to titanium is a case of balancing chargestorage capacity of the counter-electrode layer and the electrochromicefficiency of the device against the stability of the device.

Alternatively, the mixture includes vanadium, titanium and zirconium,for instance in a percentage molar ratio of 20:40:40, and such mixtureshave shown properties comparable to vanadium/titanium mixtures.

The invention also provides use of an oxide of a mixture including atleast two of vanadium, titanium and zirconium as a counter-electrodematerial for an electrochromic device.

The counter-electrode materials according to the invention may be usedin conjunction with tungsten trioxide electrochromic layers and any of arange of electrolytes, but have proven to be particularly useful forelectrochromic devices which have electrolyte layers of the compositiondisclosed in PCT/EP95/01861.

The counter-electrode materials according to the invention may alsoinclude other metals in addition to titanium, vanadium and zirconium.

An electrochromic device according to the invention is not only useablein a variable transmission window wherein the electrically conductingfilms will be translucent, but also in a variable reflection mirror (nowused particularly for automobile rear view mirrors) wherein one of theelectrically conducting films will be reflective.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of an electrochromic device according to theinvention;

FIG. 2 is a partial transverse cross section of the device shown in FIG.1;

FIG. 3 is a graph of charge storage capacity against percentage vanadiumcontent for counter-electrode layers incorporated into electrochromicdevices according to the invention;

FIG. 4 is a graph of charge storage capacity per unit thickness againstdeposition pressure for a sputtered counter-electrode layer suitable forincorporation into an electrochromic device according to the invention;

FIG. 5 is a graph of charge capacity against thickness for a sputteredcounter-electrode layers suitable for incorporation into electrochromicdevices according to the invention;

FIG. 6 is a graph of charge storage capacity per unit thickness againstpercentage oxygen in sputter atmosphere for a sputteredcounter-electrode layer suitable for incorporation into anelectrochromic device according to the invention;

FIGS. 7-11 are graphs of optical density against charge density forcounter-electrode layers suitable for incorporation into anelectrochromic device according to the invention; and

FIG. 12 is a graphical illustration of the transmission characteristicsof an electrochromic device according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIGS. 1 and 2, the device indicated generally at 1 hasfirst and second sheets of glass 2, 4 each 100 mm×100 mm, separated by a1 mm thick translucent interlayer of polymer electrolyte 6, thecomposition of which is disclosed in PCT/EP95/01861. Each of the sheetsis sputter coated on its inner face 8, 10 with an electricallyconductive film 12, 14 of indium doped tin oxide (ITO). Applied over thetop of the ITO film 12, also by reactive dc magnetron sputtering, is anelectrochromic layer 16 of tungsten trioxide, and applied over the topof the ITO film 14 is a counter-electrode layer 18. Thecounter-electrode layer 18 is applied by reactive dc magnetronsputtering as well, using an intermetallic target (not shown) ofvanadium and titanium. The target is made up in the form of an alloywith vanadium and titanium in the desired molar ratio (99.5+% pure). Thesputtering is carried out by conventional techniques in an argonatmosphere to which a small amount of oxygen is admitted. Thecounter-electrode layer 18 is laid down at a rate of 2 Ås⁻¹ to thedesired thickness. Also applied over each ITO film 12, 14, along onevertical edge, is an elongate electrical contact, commonly known as abus bar 20, 22. These are in the form of copper strips stuck on to theITO films 12, 14 with conductive adhesive. Power supply wires 24, 26 areconnected to each of the bus bars 20, 22.

The device 1 is put together as a cast-in-place laminate, using a knowntechnique. First of all the two sheets 2, 4 are formed into a cell bybonding them together (the electrochromic and counter-electrode layers16, 18 innermost) with double sided acrylic tape (not shown) between themargins of the two sheets 2, 4. Liquid electrolyte, previously degassedby stirring under vacuum, is poured into the cell. The electrolyteinterlayer 6 is then cured and the cell is scaled with an epoxy resin(not shown). The device 1 is preconditioned by cyclically driving itbetween voltages of ±3V for gradually increasing periods of time.

The device 1 is driven by applying a constant current of 10 mA (whichequates to approximately 150 μAcm⁻² for a device active area of 64 cm²)through the tungsten trioxide layer 16, the electrolyte layer 6 and thevanadium titanium oxide layer 18, via the power supply wires 24, 26 andthe ITO films 12, 14. As a protective measure, the applied voltage isnever allowed to exceed ±3V. Applying a negative voltage to the tungstentrioxide layer 16, so as to generate a current flowing in a firstdirection, causes lithium ions from the electrolyte layer 6 to beinserted into the tungsten trioxide layer 16, which produces a visibleblue coloration. Applying a positive voltage has the opposite effect,generating a current flowing in a second, opposite, direction, and thedevice 1 is bleached towards it colorless state. In the “colorlessstate” the vanadium titanium oxide counter-electrode layer is slightlycolored. Any reference herein to a positive or negative voltage can betaken also to mean a “more positive” or “more negative” voltage asappropriate, for example a device may in some instances be “colored” bydriving it from a fully bleached to a less bleached state withoutactually applying a negative voltage.

EXAMPLE 1

A device was put together as described above with a counter-electrodelayer comprising an oxide of a mixture of vanadium and titanium with thevanadium:titanium in a percentage molar ratio of 80:20. The layer 18 was3000 Å thick and was applied by reactive dc magnetron sputtering in anargon atmosphere to which 5 volume % oxygen was admitted at a totalpressure of 3×10⁻² mbar.

EXAMPLES 2-5

Devices were also made with counter-electrode layers applied under thesame conditions and to the same thickness as in example 1 except thatdifferent targets were used so as to result in oxides of a mixture ofvanadium and titanium with the vanadium:titanium in percentage molarratios of 60:40, 50:50, 40:60 or 20:80 respectively.

The charge storage capacity of each of the counter-electrode layersincorporated in example devices 1-5 was measured by a potential stepmethod with charge integration using an electrochemical interface,available in the UK from OxSys Micros Ltd, driven using softwaredeveloped by the applicants.

FIG. 3 illustrates how charge storage capacity per unit thickness of thecounter-electrode layers incorporated in example devices 1-5 increaseswith vanadium content.

FIG. 4 illustrates how charge storage capacity per unit thickness of acounter-electrode layer increases with the pressure at which it issputtered. The measurements shown are for a counter-electrode layer ofthe same composition as the layer incorporated in example device 2, thatis with vanadium and titanium in the percentage molar ratio 60:40, and 5volume % oxygen admitted during sputtering.

FIG. 5 illustrates how charge storage capacity of a counter-electrodelayer of the same composition as the layer incorporated in exampledevice 2 increases with thickness. The layer was sputtered at a totalpressure of 3×10⁻² mbar with 5 volume % oxygen admitted.

FIG. 6 illustrates how charge storage capacity per unit thickness of acounter-electrode layer with the same composition as the layerincorporated in example device 2 decreases with the volume percentage ofoxygen admitted during sputtering. The sputtering was carried out at atotal pressure of 3×10⁻² mbar.

The optical transmission of each of the counter-electrode layersincorporated in example devices 1-5 was measured by means of anelectrochemical cell (not shown) comprising electrodes submerged in aliquid electrolyte. One of the electrodes was a counter-electrode layeras incorporated into example devices 1-5 on a glass substrate.Measurements were taken using a spectrophotometer, such as thoseavailable in the UK from the Hitachi company under the designationU-4000, and by appropriately modifying the cell to gain optical accessto the counter-electrode layers. The potential applied and the injectedcurrent were controlled using an electrochemical interface, available inthe UK from the Schlumberger company under the designation 1286. In thisway, the amount of charge inserted into the layer could be accuratelycontrolled.

Initially, measurements were taken of the optical transmission of thecounter-electrode layers with no inserted charge, in the bleached state,providing the value T₀. Then, each layer was supplied with constantcurrent by the electrochemical interface for a measured time, to allowcalculation of the total charge inserted during reduction. Furtheroptical transmission values, T_(lum) were measured as the charge wasincreased and the layer became more colored. From the measurements,optical density was calculated, being related to the ratio of theT_(lum) to T₀, as a function of charge density, the total chargeinserted per unit area of counter-electrode layer.

FIGS. 7-11 illustrate how the optical density of the counter-electrodelayers of the type used in each of the example devices 1-5 variesrespectively. The rate of change of optical density with charge densityis termed the electrochromic efficiency. Thus, the electrochromicefficiency can be calculated for each counter-electrode layer from thegradient of the plots shown in FIGS. 7-11. The high vanadium contentmixture (FIG. 7) exhibits a reduction in electrochromic efficiency asthe charge density is increased. The low vanadium content mixture (FIG.11) exhibits an increase in electrochromic efficiency as the chargedensity is increased. The mixtures in between show a transition betweenthese two extremes, with the 50:50 mixture (FIG. 9) exhibiting avirtually unvarying electrochromic efficiency.

The counter-electrode optical density and electrochromic efficiencycharacteristics shown in FIGS. 7-11 are relatively low in comparison tothose of the tungsten trioxide electrochromic layer used in the exampledevices at the same charge densities, and therefore the overallelectrochromic efficiency of the device is substantially that of theelectrochromic layer.

FIG. 12 is a graph of transmission against wavelength for example device3. The transmission is measured on a scale of 0, opaque to 1,transparent over wavelengths of 240-2600 nm, which covers the visibleand adjacent parts of the spectrum. The upper curve A plots thetransmission of the device 1 in the fully bleached state and the lowercurve B plots the transmission in the fully colored state. Both curveswere measured after the device had been cyclically driven in the regionof 18000 times between fully colored and fully bleached states. Includedin the table below are values calculated using the data plotted in FIG.13 which show the difference in the luminous and direct solartransmittance (expressed as percentages) between the fully bleached andfully colored states.

Luminous Direct solar Colored 7.3 4.0 Bleached 54.1 48.2

The luminous and direct solar transmittance are calculated in accordancewith the Japanese Industrial Standard, JIS, R3106-1985.

What is claimed is:
 1. An electrochromic device having successive layersof electrochromic, electrolyte and counter-electrode materials which ischaracterized in that the counter-electrode material comprises an oxideof a mixture consisting essentially of at least two of vanadium,titanium and zirconium, wherein the counter-electrode material iscolored when reduced and bleached when oxidized.
 2. An electrochromicdevice according to claim 1 wherein the counter-electrode layer isdeposited by sputtering.
 3. An electrochromic device according to claim2 wherein the counter-electrode layer is deposited by dc reactivemagnetron sputtering.
 4. An electrochromic device according to claim 2wherein the sputtering is carried out in an argon atmosphere containingbetween 0 and 100 volume % oxygen, at a total pressure of between 5×10⁻⁴and 0.1 mbar.
 5. An electrochromic device according to claim 4 whereinthe atmosphere contains between 10 and 50 volume % oxygen, at a totalpressure of between 5×10⁻³ and 5×10⁻² mbar.
 6. An electrochromic deviceaccording to 5 wherein the target used for sputtering is anintermetallic or sintered oxide.
 7. An electrochromic device accordingto claim 1 wherein the mixture includes vanadium and titanium.
 8. Anelectrochromic device according to claim 7 wherein the oxide of themixture of vanadium and titanium is deposited by sputtering and whereinthe percentage of oxygen admitted to and the pressure of the sputteringatmosphere and the thickness to which the counter-electrode layer isdeposited are selected to provide the required charge storage capacity.9. An electrochromic device according to claim 7 wherein the vanadiumcontent of the mixture is selected to provide the requiredelectrochromic efficiency.
 10. An electrochromic device havingsuccessive layers of electrochromic, electrolyte and counter-electrodematerials which is characterized in that the counter-electrode materialcomprises an oxide of a mixture consisting essentially of at least twoof vanadium, titanium and zirconium, wherein the mixture includes two ofvanadium, titanium and zirconium in a percentage molar ratio of between10:90 and 90:10.
 11. An electrochromic device according to claim 10wherein the mixture includes two of vanadium, titanium and zirconium ina percentage molar ratio of 80:20, 60:40, 50:50, 40:60 or 20:80.
 12. Anelectrochromic device having successive layers of electrochromic,electrolyte and counter-electrode materials which is characterized inthat the counter-electrode material comprises an oxide of a mixtureconsisting essentially of at least two of vanadium, titanium andzirconium, wherein the counter-electrode layer is between 100 and 10×10³Å thick.
 13. An electrochromic device according to claim 12 wherein thecounter-electrode layer is between 1×10³ and 3×10³ Å.
 14. Anelectrochromic device having successive layers of electrochromic,electrolyte and counter-electrode materials which is characterized inthat the counter-electrode material comprises an oxide of a mixtureconsisting essentially of vanadium, titanium and zirconium.